1. Field of the Invention
The present invention relates to pulse-echo ranging and, more particularly, to a method for processing an echo amplitude profile generated by a pulse-echo ranging system
2. Description of the Related Art
Pulse-echo ranging systems, or time-of-flight ranging systems, are commonly used in level measurement applications to determine the distance to a reflective surface of a material, such as liquid, slurry or solid, by measuring how long after transmission of an energy pulse the echo or reflected pulse is received. Typically, pulse-echo ranging systems use ultrasonic, radar or microwave pulses and generally include a transmitter for transmitting the energy pulses, a receiver for receiving the echoes and a signal processor for detecting and calculating the distance or range of the material surface, based on the travel times of the transmitted energy pulses and the echoes. The transmitter and receiver may be combined in a single unit. The signal processor forms an echo amplitude profile representing the received echo amplitudes as a function of their respective travel times. Each value of the echo amplitude profile corresponds to the amplitude of an echo reflected at a certain distance from the transmitter/receiver.
One main problem of echo processing is selecting the wanted level echo from other multiple echoes that may occur at short ranges, when measuring shallow liquid levels or by reflections from obstacles in the measuring environment such as the internal structure of and installations in a tank or vessel containing the material. The echoes may overlap and may be further distorted by energy lost, delay or noise corruption.
It is thus a challenge to identify the echo of interest that corresponds to the material. It is also important to assess a level of confidence that the echo selected is indeed the echo of interest.
A commonly used technique for finding valid echoes in an echo amplitude profile involves generating a time-varying threshold (TVT) function. The TVT function provides a baseline on the echo profile indicating the level of noise and the presence of obstacles. Various algorithms and techniques are known for determining a noise floor and generating the TVT function. Valid echoes appear above the TVT function. Based on a variety of criteria (e.g., time of arrival, amplitude, amplitude and/or area above the TVT function), each echo is rated and attributed a level of confidence. The echo with the highest level of confidence is selected as the echo of interest.
However, especially in the case when multiple echoes are overlapped, it is impossible to accurately estimate the level based on merely a single echo. Instead, the whole picture would be available if the times-of-flight of all of the echoes or at least all potentially relevant echoes are estimated, and that would assist in an accurate level estimation using other known properties on the physical environment.